This invention relates to the detection of defects in semiconductor materials and devices.
Electron beam induced current, also known as electron beam injected carriers (EBIC) microscopy is a known analysis tool for detecting defects or junction integrity in semiconductor devices such as lasers, photodiodes, photodetectors, FETS, and MOS devices. In the prior art, a scanning electron microscope (SEM) irradiates the semiconductor device under observation with an electron beam which produces electron-hole pairs within the device. By detecting the signal induced in the semiconductor in response to the incident beam as it scans the semiconductor device and using the induced signal to modulate the signal intensity on a cathode ray tube (CRT) an image of the semiconductor device is derived from which material properties and/or defects of the device can be observed.
Prior art methods and apparatus for detecting the signal induced in the device in response to an incident electron beam have employed a current sensitive preamplifier or voltage sensitive preamplifier to detect the induced current or voltage, respectively. The voltage output of either type of preamplifier has then been used to modulate the intensity of a CRT video signal on a one-to-one linear relationship with the induced signal as the beam scans the device. Disadvantageously, semiconductor devices biased at their normal operating bias current levels could not be examined since the bias current would also be fed to the preamplifier and overwhelm the induced signal. Since certain material and/or manufacturing defects are bias dependent, these defects could not be readily detected using these prior art techniques. Furthermore, alternative prior art techniques for analyzing biased semiconductor devices have provided unsatisfactory results. In one arrangement, the bias current is blocked from the current sensitive preamplifier by means of an ac coupling capacitor. However, the resultant output of the preamplifier will no longer be proportional to the induced current thereby making detection of defects difficult. In a second arrangement, dc offset current at the input of the preamplifier is used to offset the bias current through the device. The offset current will not however offset the noise associated with the bias current and the resultant image derived from the preamplifier output is a noisy representation of the device from which defects cannot be readily detected.
Another problem associated with these prior art defect detection techniques is that they cannot be used for defect detection of MOS type devices which are a large percentage of semiconductor devices presently being manufactured. In particular, the magnitude of the irradiating electron beam causes holes to be trapped within the oxide layer of MOS devices thereby inducing a threshold shift which modifies the device behavior. In order to remove these modifications, a high-temperature annealing process must be performed on the modified MOS device.